1. Field of the Invention
The present invention relates to a method for producing an excellently strong thin wall cordierite ceramic honeycomb structure by extruding.
2. Description of Related Art
Cordierite ceramic honeycomb structures are used as an exhaust gas purifying catalyst carrier, a filter, or a heat exchanger for automobiles or the industrial use.
Recently, improvement of the thermal shock resistance and the strength have been desired particularly for a honeycomb catalyst carrier to be used in an exhaust gas apparatus for automobiles since it needs to be mounted in the vicinity of the engine in order to improve the catalyst performance by reducing the volume of the catalyst carrier, to improve the light off performance, to lower the pressure loss for improving the fuel consumption performance and improving the output, to improve the strength for reducing the cost for canning into the casing, and to improve the catalyst activity.
Therefore, a thinner wall for a rib of a honeycomb structure for improving the catalyst performance and a thinner wall for a rib of a honeycomb structure while maintaining the cell density for achieving a higher cell density and a lower pressure loss have been conventionally discussed. However, since materials (in particular, magnesia material) needs to have fine particles due to the strength decline for the thinner wall in the porous cordierite honeycomb structure and reduction of the die slit width at the time of extruding, a problem is involved in that a drastic rise in the thermal expansion ratio can be generated.
Furthermore, it is difficult to have cordierite ceramics denser. In particular, since the amount of impurities to be a fusing agent such as calcia, alkaline, and sodium carbonate needs to be extremely small with a low expansion cordierite material showing a 2.0.times.10.sup.-6 /.degree.C. or less at a room temperature to 800.degree. C. coefficient of thermal expansion, the glass phase becomes extremely small so as to form a porous substance.
In particular, since a cordierite honeycomb structure recently used as an automobile exhaust gas purifying catalyst carrier requires a 1.5.times.10.sup.-6 /.degree.C. or less coefficient of thermal expansion from a room temperature to 800.degree. C., the porosity can be 20 to 45% at best even if refined talc, kaolin, alumina materials with little impurities. In particular, in a honeycomb structure having a 30% or less porosity, increase of the impurity amount and finer material particles are necessary so that one having a 1.0.times.10.sup.-6 /.degree.C. or less coefficient of thermal expansion from a room temperature to 800.degree. C. have not been obtained.
Furthermore, since a cordierite honeycomb structure having a comparatively low porosity contracts drastically in the drying and firing processes, a crack can be easily generated and thus it is difficult to obtain a large honeycomb structure with a good yield.
In order to solve the problems, Japanese Patent Publication (Kokoku) No. 4-70053 discloses a method of improving the strength of ceramic itself by having the porosity of cordierite ceramics denser to 30% or less.
In the method, the ceramic itself becomes denser and stronger by having a 30% or less porosity of the cordierite ceramics in order to prevent decline of the isostatic strength, which is the compression load from the outer wall and outer periphery direction of the honeycomb structure by the honeycomb cell deformation generated at the time of extruding.
However, in the above-mentioned method, since the porosity of the cordierite ceramics is 30% or les, the improvement of the catalyst carrying characteristics of the cordierite honeycomb structure cannot be expected, and further, the effect of having a thinner wall and a higher cell density in the cordierite honeycomb structure was not sufficient.